This invention relates to the brazing or sintering of particulate materials and, more specifically, to the materials used for wide-gap joining, repair or surface coating of gas turbine components.
Gas turbine components, such as superalloy blades and vanes, are subjected to high temperatures and stresses during engine operation. Under such conditions they will often become physically damaged due to the formation of cracks, voids and worn surfaces. When the damage extends beyond certain allowable limits, a decision must be made to either repair or replace the components. Because they are expensive to manufacture, there is considerable economic incentive to attempt repair of turbine components by methods such as welding, brazing or wide-gap brazing.
Wide-gap brazing refers to the repair of defects too large to be filled or bridged by standard brazing techniques wherein the gap filler material is drawn into defects by capillary forces alone. Therefore, wide-gap filler materials must to be physically pre-placed within joints or defects or onto surfaces and, during heat treatment, exhibit sluggish flow which prevents them from substantially flowing out of the repair area. Prior art wide-gap filler compositions are typically comprised of a mixture of superalloy and braze alloy powders suspended in some type of temporary organic vehicle so as to form a slurry, paste or transfer tape. The organic vehicle, or binder, is usually comprised of an organic polymer dissolved in a solvent, and sometimes includes a plasticizer and dispersant. The organic polymer provides strength to the alloy powder deposit after the solvent has evaporated, bonding the powder particles to each other, as well as to the substrate article. (The word xe2x80x9cbinderxe2x80x9d is usually used to mean all of the ingredients present in the vehicle, including the organic polymer, solvent, plasticizer, wetting agent, etc. However, in many references, the word xe2x80x9cbinderxe2x80x9d refers specifically to the organic polymer constituent of the vehicle. When used herein, the word xe2x80x9cbinderxe2x80x9d shall be used in the traditional sense and the phrase xe2x80x9cprinciple binder resinxe2x80x9d shall be used to refer to the organic polymer component.) Subsequent drying and furnace heat treatment operations decompose and vaporize the various binder constituents, followed by brazing or sintering of the powder. Alloy powders used for wide-gap filler materials are described in U.S. Pat. Nos. 4,073,639 and 4,381,944. Organic binders and methods used in the formulation of slurries, pastes and transfer tapes have been described in U.S. Pat. Nos. 2,908,072; 3,293,072 and 3,589,952.
Historically, wide-gap repairs were developed for the repair of defects in aero or aeroderivative gas turbine components. Relatively speaking, these components and the defects in them tend to be small. For example, a typical wide-gap crack in an aero gas turbine component might be about xc2xc inch in length by about 0.030 inches in width or depth. In contrast, heavy frame gas turbines which are designed primarily for industrial power generation are much larger than aero or aeroderivative gas turbines. A single vane segment or blade from one of these engines can weigh upwards of 100 lbs. Crack defects in these components are correspondingly much larger, with dimensions often exceeding several inches in length and up to one inch in width or depth. Standard welding techniques cannot always be used to successfully repair this type of damage, and it is again desirable to be able to use some type of wide-gap repair process for component restoration.
While the wide-gap slurries, pastes and transfer tapes of the prior art have been found useful for the repair or joining of the smaller areo components, there are many situations in which these materials are unsatisfactory for the repair of larger defects in heavy frame gas turbine components. For instance, it is often desirable to be able to apply the wide-gap filler material to thicknesses of xe2x85x9 inch or more onto surfaces with vertical or inverted orientations. After it is applied, the filler material should neither flow, shrink, nor form defects such as voids, tears and the like during subsequent handling and heat treatments. Prior art wide-gap repair materials will either slump or fall off the article during drying and/or heat treatment when used in this way, making it necessary to complete the brazing or liquid phase sintering operation in a number of steps by varying the orientation of the article in the furnace each time.
Some additional requirements of a good wide-gap filler material are that, during its initial application, it should be capable of plastic flow together with adhesive properties which are similar to those of a modelling clay. These properties would allow the alloy powder mixture to flow into a desired shape by applying a moderate force, for example by hand, and thereafter keep its shape, while adhering to the substrate article in various orientations. Once the external force is removed from the wide-gap filler, it should keep its shape while the repair article is handled, stored, dried, and heat treated. These attributes are not found in the wide-gap slurries, pastes and transfer tapes of the prior art. For example, a powder metallurgy repair material, comprised of a mixture of iron-base alloy powders and a plastic binder, has been described in connection with the repair of centrifugal pump impellers (Welding Journal, April 1971, pp. 255-256). The proprietary materials used in this method were molded by hand, however, back-up supports were needed on the underside of through-going defects to hold the powder mix in place. In other words, the mixture was not self-supporting.
Still another limitation of prior art wide-gap filler materials has been encountered in the repair of hollow gas turbine components which contain through-going defects or details. In most cases, drop-through or flow of the repair filler material into interior cooling passages or cavities cannot be tolerated, since obstruction of these passages would render the component unserviceable and unrepairable due to the limited access afforded by the component design. It is very difficult to control the flow of prior art wide-gap pastes and slurries which makes them unsuited to the repair of these types of defects or details. An important advantage of the wide-gap filler material of the present invention is that the aforementioned limitations related to molding, flow, slumping and loss of adhesion can be eliminated. This advantage is realized through the use of the novel sacrificial binder system of the present invention.
Finally, within the general category of materials comprised of metal powder alloys and organic binders there exists another class of materials which are used in the powder injection molding art. Powder injection molding (herein referred to as PIM) is a method for the fabrication of ceramic or metallic sintered parts. A solid green body or compact comprised of a ceramic or metallic particulate material and a sacrificial binder mixture is molded in a die by the application of heat and mechanical pressure in an injection molding machine. The binder ingredients are later removed from the green body in a series of solvent or thermal debinding processes, followed by firing and sintering of the compact. The main PIM binder types are thermoplastic, thermosetting and gelation systems (R. M. German, Powder Injection Molding, Metal Powder Industries Federation. Princeton, N.J., 1990, pp. 99-124). Thermoplastic sacrificial binders used in the formulation of PIM feedstock are rigid and non-adhesive at room temperature and must be softened by heating before the mixture will flow adequately to allow mold filling. Thermosetting and water-based gelation binders develop their strength by cross-linking of the polymer units at elevated temperatures. Rigid, self-supporting compacts can only be produced from these materials by heating the die cavity after the feedstock has been introduced. The need for substantial temperature and pressure variations during the processing of PIM feedstocks makes the binders used in these formulations unsuitable for use in conjunction with the wide-gap filler materials of the present invention.
One object of the present invention is to provide an improved wide-gap filler material, useful for the manufacturing, joining or repair of metallic articles which will allow the positioning and heat treatment of thick (e.g. xe2x85x9 inch), complex, near-net-shape powder alloy deposits in horizontal, vertical or inverted orientations without the need for surrounding back-up dams or support materials.
Another object is to provide a wide-gap filler material which, in its initial form, will remain soft, moldable, self-supporting and adhesive (tacky), even after being exposed to air at room temperature over a period of several hours.
Still another object is to provide a wide-gap filler material which will not slump, shrink, crack or flow away from its initial position during subsequent handling or vacuum heat treatment operations, even when it is positioned in vertical or inverted orientations.
A further object is to provide a wide-gap filler material which can be used to fill through-going defects in hollow components where there is limited or no access to the interior cavity. The improved wide-gap filler material of the present invention will not slump or drop through into the interior cavities of the component during application or processing.
These and other objects and advantages will be more fully understood from the following detailed description of the preferred embodiments, which are intended to be typical of, rather than in any way limiting on, the scope of the present invention.
Briefly, in one form of the present invention there is provided an improved sacrificial binder mixture consisting essentially of, by weight, 31-33% acrylic resin, having a glass transition temperature (Tg) less than 20xc2x0 C., 22-24% phthalate or adipate-type plasticizer, 42-44% glycol ether or glycol ether acetate with a vapor pressure of less than 20 mm Hg at 25xc2x0 C. and 1-2% nonyl phenol base or octyl phenol base nonionic surfactant as a wetting agent. The binder mixture is combined with a finely divided alloy powder or blend of powders to give a composition consisting of, by weight, 5-8% of the binder mixture and 92-95% of the alloy powder or powders. The resulting filler material is in the form of an adhesive, self-supporting, moldable putty, capable of being forced under manually applied pressure to bond with, and take the shape of, a joint or repair cavity in any orientation.
In another form of the present invention, there is provided a two-part (Part A and Part B) wide-gap filler material comprised of two powdered alloys, provided as separate components, each in the form of a moldable, adhesive putty. The first component (Part A) putty comprises a metal alloy powder having as its basis metal an element selected from the group consisting of nickel, cobalt or iron. The first component putty may, in addition, contain a certain amount of a non-metallic (e.g. oxide, nitride or carbide) particulate material. The second metallic alloy powder has a liquidus temperature lower than the solidus temperature of the first component particulate material(s) and the substrate article. The ratio of these two putty components, Part A (containing the first alloy powder(s) plus binder mixture): Part B (containing the second alloy powder plus binder mixture), is controlled to be in the range from 4:1 to 1:1, depending on the properties required in the final repair deposit and the processing characteristics of the alloy powders when they are used to repair or join the metal articles.
The invention in still another form provides a method for using the putty containing the improved sacrificial binder of the present invention to manufacture or repair a metallic article. After preparation and cleaning of the bonding or mating surfaces, a single or two-part alloy putty is applied to the article and shaped or molded to produce a near-net-shape build-up. The sacrificial binder ingredients are then removed in a controlled thermal process to prevent flow, slumping, or separation of the repair deposits from the article. The sacrificial binder ingredients are additionally removed by methods which allow vapors and gaseous decomposition products to escape from the wide gap filler material without causing internal pressure build-ups which could otherwise lead to the formation of defects such as voids, blisters, cracks, tears and the like.
This application also relates to a kit for the composition and method for repair.